May 1, 1895.] 



KNOWLEDGE. 



117 



navigating this dangerous seaway, and its no less perilous 

 offsets. 



Kouglily speaking, the Baltic Sea may be regarded as a 

 series of submarine pUteaux. hollows and channels, 

 separated by shallow ridges. In some of these deeper 

 depressions or pools there is little, if any, circulation of 

 the water. Over the shallow ridges, however, there is 

 abundance of movement, and that, toi, of a most complex 

 character ; for over these the incoming waters from the 

 North Sea and the outflowing waters from the Baltic 

 must flow. 



The rivers which pour their tributary waters into this 

 sea are numerous. Tuere is in addition a copious rainfall 

 upon its surface, and, as may be imagined from the 

 general climatic conditions which obtain over the Baltic 

 area, the evaporation is by no means great. Tuere is thus 

 an excess of supply over evaporation. Tue surplus resultant 

 finds its way into the North Sea through the channels 

 already alluded to, and by the same means of communica- 

 tion m'lst enter the warmer and salter currents from the 

 adjoining German Ocean. It is this minghng of the 

 waters which is accountable for the ground, anchor, or 

 pancake ice of the Baltic and its connections. 



The Baltic stream, as the outflowing waters are styled, is 

 thus a creep of colder and fresher water than that which 

 obtains in the adjacent ocean. In winter the rivers are ice- 

 bound and the flow of the stream diminishes. Iq February 

 or March, when the advent of warmer weather releases the 

 waters of the icebound streams, the flow assumes again its 

 normal proportions. 



This superposition of layers of water of different 

 densities and different temperature plays, duriag the 

 winter months, an important part in the production of 

 what is, in some cases at least, erroneously termed ground 

 ice. Tiie phenomenon of ground ice is no recent discovery. 

 Long ago it was matter of pretty common observation 

 that, under certain circumstances, ice was formed at the 

 bottom of sheets of water, more especially of rivers. The 

 explanation as to the formation of this variety of ground 

 ice is a simple one. It is well known that running water 

 does not freeze so readily as that which is stUl, so that it 

 is quite possible, by reason of the constant movement 

 among the particles cf water in a rapidly flowing stream, 

 that the temperature of this water may be reduced con- 

 siderably below the freezing point before ice is formed. 

 It is easy to imagine water in this condition transferred, 

 by the mechanical action of an on moving stream, from 

 surface to river bed. Here, at the bottom, ice is formed 

 owing to the greater stillness of the water, and the fact 

 that rocks, stones, and shingle ofl'er so many points upon 

 which the water can freeze. An increase in the bulk of 

 this ice, or the raising of the temperature, and the conse- 

 quent reducing of the density of the water flowing over it 

 ensures that the buoyancy of the ice asserts itself, and the 

 subaqueous formed ice floats upon the surface. Such ice 

 always carries with it the marks of its ground origin in 

 the shape of the stones, gravel, mud or weeds which are 

 frozen into its under surface. 



The so-called ground ice of the Baltic is not of this 

 type. It is more analogous to that which came under the 

 observation of Captain Scoresby and other early Aixtic 

 navigators. They reported ice as observed in the very act 

 of rising to the surface from the depths of the sea, at 

 considerable distances from the Greenland coast. Cakes of 

 .this ice, when carefully examined, show no siga of earthy 

 contact on their under side. It is really sea ice crystallized 

 not on the surface of the sea, but in it. 



The suggestion that sea ice in the Polar regions or the 

 Baltic Sea should be formed anywhere but at the surface 



was at one time looked upon with open disbelief. The 

 main diffii^uUy in the way of accepting what at first seems 

 so contradictory a phenomenon lies in the fact that with 

 fresh water the point of greatest density is reached some 

 seven degrees sooner than the freezing point. Thus, in 

 ponds and lakes where the conditions are normal, the 

 coldest water after 39' Fahr., the temperature of the point 

 of maximum density, has been passed will be the lightest, 

 and hence surface-freezing ; but these conditions are not 

 applicable to the crystalUzation of fresh or salt water 

 under all circumstances. 



Perhaps the Baltic Sea and its offsets furnish the best 

 and most interesting example of the so-called ground ice 

 of the sea, upon a big scale. The freezing point of the 

 water composing the Baltic stream is, of course, hiijher 

 than that of the Salter water which flows into the Baltic, 

 mainly as an under-current from the North Saa. The 

 temperature of this underlying stratum may be below the 

 freezing point of the fresher water which is superiacum- 

 bent to it. When this is so it is easy to suppose the 

 fresher water, chilled by contact with a keen and nipping 

 air to its point of maximum density, sinking slowly down- 

 wards to the colder and Salter stratum beneath. Here 

 the temperature is such that it is speedUy reduced to the 

 freezing point, and cakes of ice are actually formed at the 

 point of contact of the two layers of water. Of course, the 

 formation of ice in this manner in the intermediate layers 

 of the sea is influenced also by the eddies which the 

 mixing of such currents as these must produce. 



The winter of 1892-3 will long be remembered on account 

 of the damage inflicted by ice upon shipping in the Baltic 

 and i:s approaches. Large and staunch vessels were 

 prevented from proceeding by the rapidly formed and 

 erroneously named ground ice. The cakes of ice no sooner 

 reached the surface than the process of regelation cemented 

 the fragmentary ice into a compact field. Fishermen, 

 plying their necessary calling at no great distance from the 

 shore, saw the clear belt of water become quickly covered 

 with an ever-growing expanse of ice which cut off all 

 chance of navigating their %-essels to land. Iq many cases 

 the pack in which the fishers' boats, the sailing craft, or 

 the smaller steamers became involved, drifted out to sea, 

 where the vessels were crushed, their occupants perishing 

 miserably. 



5omt iicccut patents. 



J. Gr. Stidder, Croydon. Ai improved method of for ning the 

 joints of pipe* f jr steam, water, and other fluids. Figs. 1 and ^show 

 the ins-, ntion, in elevation and section, a is ne end of a socket into 

 which the pipe b is screwed, c is an annular rece-s formed in the 

 socket, in which a ring of a>be-tos. copper lead, rubber, cord, or other 

 suitable material d is inserted, with or without the addition of red 

 lead and the like, as rec^iired. e is a screw collar or back nut, also 

 formed with an annular recess and a projection/ whicti enters the 

 recess in a, a short d -tance. The back nut is screwed up until the 

 joint ii perfectlv fluid tight. 



Four fiffui-ei'. Dated 2ofh April. 1S94. Accepted 2nd March. 

 1895. yo. 802 J. 



fuj 1 



n^ 2 



